In recent years, due to miniaturization and multi-functionality of mobile communication terminals, relays, and base stations, many studies on improvement in the efficiencies of RF power amplifiers that consume almost all consumption power are being carried out in an effort to increase battery time, and, among them, studies on Doherty power amplifier, which is representative of solution for increasing efficiency of a RF power amplifier, are mainly carried out.
The Doherty amplifier was provided by W. H. Doherty in 1936, in which a carrier amplifier and a peaking amplifier are connected in parallel to each other using a quarter wave transformer (λ/4 line). The Doherty amplifier varies the amount of current supplied to a load by the peaking amplifier according to power level to control the load line impedance of a carrier amplifier, thereby increasing efficiency thereof.
A Doherty power amplifier for used in a handset, i.e. a Doherty amplifier for a mobile communication terminal according to the prior art replaces a quarter wave transformer by a circuit such as a π-network including an inductor L and a capacitor C. Input matching circuits are connected to input terminals of the carrier amplifier and the peaking amplifier, and output matching circuits are connected to output terminals of the carrier amplifier and the peaking amplifier.
In realization of Doherty power amplifiers for mobile communication terminals, due to increased importance of their size, they are needed to be miniaturized, in which case the Doherty amplifiers are being realized on PCB packages due to their sizes and inductor losses in spite of their needs for integration on chips.
In order to widen the bandwidth of a frequency and obtain broadband characteristics, reference impedance is generally matched to desired impedance through a matching circuit conventionally including two or three inductors, a capacitor, and/or a micro-strip line. In this case, the circuit structure becomes complex as the number of matching circuits increases, causing the sizes of chips and circuits to become larger and causing increase in cost owing to additional use of passive devices. Then, as the number of passive devices increases, the matching circuit loss in the entire circuit increases due to loss in the passive devices, thereby causing lowering of the efficiency of the circuit.
FIG. 1 is a circuit diagram illustrating the first example of a Doherty power amplifier according to the prior art, and FIG. 2 is a circuit diagram illustrating an example of widening the bandwidth of a frequency of the Doherty power amplifier in FIG. 1. The reference numeral 11 in FIGS. 1 and 2 denotes a carrier amplifier, the reference numerals 13, 14, 23, and 24, output matching circuits, the reference numeral 15, a micro-strip line, and the reference numeral 21, a peaking amplifier. The micro-strip line 15 functions as a quarter wave transformer. A load impedance RL and a characteristic impedance R0 do not need to be 50 Ohm and 100 Ohm, respectively, but need to satisfy the condition of R0=RL×2. FIG. 2 realizes a process of matching the output impedances of the carrier amplifier 11 and the peaking amplifier 21 from the load impedance RL to an optimum power source impedance Ropt of a power device with two-section output matching circuits 13, 14, 23, and 24 to thereby widen the bandwidth of a frequency.
FIG. 3 is a circuit diagram illustrating the second example of a Doherty power amplifier according to the prior art, and FIG. 4 is a circuit diagram illustrating an example of widening the bandwidth of a frequency of the Doherty power amplifier of FIG. 3. In FIGS. 3 and 4, the reference numeral 31 denotes a carrier amplifier, the reference numeral 33, a micro-strip line, the reference numeral 41, a peaking amplifier, the reference numeral 43, an offset line, and the reference numerals 51 and 53, output matching circuits. The Doherty power amplifier of FIG. 3 has a configuration that is advantageous in view of miniaturization and integration by lowering the characteristic impedance of the micro-strip line 33 functioning as a quarter wave transformer, and FIG. 4 represents a structure for widening the bandwidth of a frequency by realizing a process of matching output impedances of the carrier amplifier 31 and the peaking amplifier 41 from a load impedance RL to an optimum power source impedance Ropt of a power device with two-section output matching circuits 51 and 53.
FIG. 5 is a circuit diagram illustrating the third example of a Doherty power amplifier according to the prior art. The reference numeral 61 of FIG. 5 is a carrier amplifier; the reference numerals 63, 73, and 83 are output matching circuits; the reference numeral 65 is a micro-strip line functioning as a quarter wave transformer; the reference numeral 71 is a peaking amplifier; and the reference numeral 75 is an offset line. In FIG. 5, the one-staged output matching circuits 63 and 73 among the two-section output matching circuits 63, 73, and 81 are used for widening the bandwidth of a frequency and are located on the output lines of the carrier amplifier 61 and the peaking amplifier 71. The Doherty power amplifier of FIG. 5 has almost the same characteristics as that of FIG. 4.
The first to third examples of FIG. 2, FIG. 4, and FIG. 5 use two-section output matching circuits to widen the bandwidths of frequencies, thereby obtaining the broadband characteristics, in which case the structures thereof are complex, causing large sizes of chips and circuits, and the matching circuit loss of all the circuits increases due to the losses in passive devices, causing lowering of the efficiencies of the circuits. In particular, the first example of the prior art is vulnerable in integration and miniaturization due to a high characteristic impedance of a micro-strip line functioning as a quarter wave transformer when it is used as a power amplifier for a mobile communication terminal.
Further, even when a Doherty power amplifier is employed on a package of a printed circuit board in order to realize a power amplifier for a mobile communication terminal, the size of the package and the manufacturing cost increase due to the use of several passive devices.
In addition to disadvantages in view of miniaturization of such a power amplifier, the broadband characteristics of a power amplifier may limit the adoption of power amplifiers that are to be used in the next generation mobile communication system.